1. DETERMINATION OF THE TOPOLOGY OF HIGH SURVIVAL HF RADIO COMMUNICATION NETWORK Andrea Abrardo.

2. Outline 2SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 1.Communications requirements 2.The physical topology of connections envisaged in the SWING project 3.Higher layer requirements 4.Identification of MAC techniques and possible scenarios 5.Comparisons 6.Looking forward

3. Communications requirements ECIs send data only to their home CGAs (ECIs to ECIs communication is not required, not even within the same area). More than one ECI belonging to the same area, i.e., connected to the same CGA, may transmit at same time. Communication between different CGAs must be permitted. The major part of network traffic is confined within each area, i.e., ECIs to CGAs communications. Accordingly, the probability of concurrent transmissions of ECIs (either belonging to the same area or not) is greater than that of CGAs. 3SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

4. HYBRID TREE/MESH TOPOLOGY 4SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 The choice of the topology has a direct impact on the design of higher layer protocols, and vice versa.

5. Higher layers requirements SWING requirements involving higher layer operations – Most of the times end-to-end communications are confined to single-hop communications, i.e., from ECIs to CGAs or among CGAs (MAC layer aspects) Considering these premises we provided: – MAC requirements – Identification of possible MAC techniques suitable for the SWING system – MAC protocols comparison 5SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

6. MAC requirements Contention-based MAC protocols such as IEEE 802.11 provide fast and efficient channel access in presence of light traffic. Contention-free protocols such as IEEE 802.4 (token bus) achieve high efficiency under heavy loads, and provide bounded access time as well. One of the factors that influence the performance of MAC protocols for HF networks is the link turnaround time, i.e., the time it takes for a node to process a received packet and to send back a response. 6SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

7. Identification of possible MAC techniques: Token Passing Token passing is a contention-free protocol where the nodes agree that only the node that currently holds a token is allowed to transmit. The HF Token Protocol (HFTP) is a token bus protocol, so each node can (nominally) send data directly to any or all other nodes (broadcast channel). Token passing is attractive for use in surface-wave naval high-frequency (HF) radio networks due to its potential for high throughput, fairness, and bounded access time. 7SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

8. Identification of possible MAC techniques: Distributed Coordination for High Frequency (DCHF) DCHF is a contention-based scheme based on the popular wireless MAC protocol IEEE 802.11 DCF. The main characteristics of DCHF are the lack of synchronization requirement and the possibility left to the nodes to join or leave the network, without any management overhead. DCHF uses only "virtual carrier sensing“ implemented through the exchange of RTS and CTS packets. 8SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

9. Possible scenarios Considering the general comms requirements, and the characteristics of the two MAC schemes we propose 3 different solutions based on 3 different scenarios: 1.Token passing scenario: in this solution the logical ring is passed among all the nodes of the SWING network (both among ECIs and among CGAs). 2.Contention based scenario: all nodes belonging to SWING network apply DCHF MAC technique; every transmitting node senses the channel and then establishes a connection with the receiver. 3.Mixed scenario: in this case, we have a contention free access technique (i.e., HFTP) for inter-area communications, i.e., between ECIs and their home CGA, while for the global network (communication between CGAs) we consider a contention based access technique (i.e. DCHF). 9SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

10. Token Passing scenario This solution allows every radio terminal to transmits its data without incurring in collisions. In case of heavy traffic, this procedure helps regulating the access to the channel avoiding collisions. On the other hand, for light traffic (i.e. only one ECI want to transmit to the respective CGA) this solution introduces a strong management overhead, which severely degrades the system performance. Do not recommended in presence of long turnaround times (due to long interleavers). 10SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

11. Contention based Scenario Suitable for light traffic (no collisions). In presence of long turn around times it is necessary to avoid RTS/CTS: this is possible if each node lies in the interference region of each other (channel sensing is sufficient to avoid collisions). 11SWING Final Meeting | CNIT - Pisa, Italy13/12/2013

12. Mixed Scenario Whenever an ECI have to transmit a packet to its home CGA, it waits for the reception of the token that is continuously exchanged within the logical ring composed by one CGA and its connected ECIs. When a CGA receives the token, it starts the contention based procedure to submit a pending packet towards another CGA. In case no pending packet is present, the token is released, otherwise it is kept until packet transmission. 12SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 Such a solution could efficiently manage the tradeoff between access time and system throughput.

13. Comparisons (Simulations) Comparisons 13SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 Behavior of MAC techniques with turnaround of 1 s. Top: average latency per packet in DCHF (red) and HFTP (blue); Down: channel utilization in DCHF (red) and HFTP (blue). Simulated scenario

14. Comparisons (Simulations) Comparisons 14SWING Final Meeting | CNIT - Pisa, Italy13/12/2013 Throughput versus turn around: saturation traffic analysis.

15. Looking forward Include MAC protocols in the SWING system. Network layer issues: – In the present SWING scenario, network functionality can be regulated as a transparent bridge, without the need of additional overhead due to IP addresses. – However, scaling to larger networks with more complex topologies needs the introduction of routing algorithms. 15SWING Final Meeting | CNIT - Pisa, Italy13/12/2013